1. Field of the Invention
The invention relates to a motion detection system for display cases.
2. Description of the Prior Art
Passive infrared (PIR) detectors are often used to trigger illumination. Retail stores, particularly grocery stores, store goods in cases. Some of the upright refrigerated cases have doors on them, others are chest-type refrigerated cases, which are open and readily accessible from above. Often, these cases are constantly illuminated, to better display the goods stored within. For reasons of economy, store owners or operators now frequently desire to keep the illumination turned off, except when a customer approaches the case. There are several known motion detection devices that use PIR motion sensors to control the illumination of display cases. One uses a single PIR motion detector sensor 1 placed centrally on the front face of the case. See FIG. 1. The field of detection or field of view fans out in a cone shape from the sensor and leaves large areas toward the side edges of the case uncovered by the field of view, as shown by the polar plot. The polar plot indicates the sensitivity of the detector, on a scale of 1 to 10, 10 indicating maximum sensitivity. The large areas uncovered by the field of view are null areas N and are less than 5 on the sensitivity scale. The device also detects motion that is too far away from the case, because the greatest sensitivity is in a normal direction to the case. The field of detection is limited to 60 degrees and does not provide 180 degrees field of view, which is required in order to accurately “see” a customer approaching from the sides of a refrigeration case.
A second device uses two PIR motion detector sensors 2 located centrally on the front of the case C, with their axes angled 65 degrees azimuthally to provide a field of detection that is approximately 140 degrees. See FIG. 2. The field of detection from the two sensors covers only partially the central portion of the case, as indicated by the null area in the detection coverage, and also has null areas on either side of the case. Assuming that refrigeration cases are arranged close to each other in rows with the front of the cases of two rows facing each other across an aisle, the dual-sensor device on one case may detect a customer who is approaching a case on the other side of the aisle, yet will fail to detect a customer approaching close to the side of the case. If the device is used on a case that is located at the end of the aisle, such that customers typically approach it on a path that is close to being normal to the case, the device will not trigger until the customer is relatively close. Also, because of the pronounced sensitivity at 30 and 330 degrees, the device will trigger unnecessarily when customers are detected at a greater than desired distance.
Motion detection lighting systems are typically subjected to large numbers of activation cycles during their lifetimes and the number can often exceed the estimated life of a mechanical relay. Some of these lighting systems have ballasts or power supplies that have a high in-rush starting current. This repetitive high starting current decreases the life of the mechanical relay and the power supply.
What is needed is a detection system that detects motion in the front of a display case adequately, without unnecessarily detecting motion off to the side of the case or too far away in front of the case. What is further needed is such a system that avoids null areas in a desired and foreseeable approach path and avoids detection beyond the side edges of the case. What is yet further needed is a device that is simply and quickly incorporated into existing circuitry. What is still yet further needed is such a system with improved reliability.